On some Properties of Light. 201 
atose from the existence of a double dispersive power. 
This conclusion was confirmed by determining the disper- 
Sive power of the greatest refraction, which coincided ex- 
actly with the order assigned to it by Dr. Wollaston. 
The dispersive powers, which I have obtained for other 
doubly refracting crystals, such as carbonate of strontites, 
carbonate of lead, and chromate of lead, have confirmed 
this result, and establish the general law, that each refrac- 
tion of crystals which give double images is accompanied 
with a separate dispersive power. The double dispersive 
powers of these bodies are given in the following table. 
Chromate of lead (gr. refr.) estimated at 0°400 
Ditto Ditto must exceed 0°296 
Ditto (least teh) 1 s)sb e:eicinn 4. 101262 
Carbonate of lead (gr. refr.).....0-... 0 091 
Ditto (least seis) seine tea) OF 066 
Carbonate of strontites (gr. refr.) .....  0°046 
Ditto (least refr.)....  0°027 
Calcareous spar (gr. refr.) ...2.2..+..  0°040 
Ditto: + (.(least refi), ./tisa/su) 2, 7 0026 
In a table of refractive powers, published by the late Mr. 
Cavallo, he has given, from other authors, the dispersions, 
or the dissipations as he calls them, of a few substances, 
and he has annexed a different dispersion to the two refrace 
tions of Iceland crystal; but it is obvious, from a simple 
inspection of the table, that these are measures of the di- 
Spersion or quantity of colour, and not of the dispersive power 
of the substances. The measures in the table alluded to, 
with the exception of one or two, are so completely in- 
compatible with those taken by Dr. Wollaston and myself, 
that I can scarcely believe that the experiments were ever 
made. 
The singular property of a double dispersive power, while 
it seems to exclude some of the theories by which the donble 
refraction has been explained, adds another to those nu- 
merous difficulties with which philosophy has yet to strug- 
gle, before she can reduce to a satisfactory generalization 
those anomalous and capricious phenomena which light 
exhibits in its passage through transparent bodies. 
I have the honour to be, dear sir, 
Your most obedient humble servant, 
Edinburgh, 23, Duke-street, Davip BauwsTer. 
December 19, 1812. 
To Sir H, Davy, LL.D. F.R.S. 
T2 L. On 
